The present invention relates in general to a control apparatus and method for controlling a variable electric load which is supplied by a conventional power grid and a non-controllable, renewable power supply such as a photovoltaic power source, and more particularly, to a load controller and method which seek to maximize effective capacity of the renewable power source.
The practical use of photovoltaic systems, which convert incident sunlight into electrical energy, continues to be of significant interest to the power generation industry. The development of more efficient solar cells, as well as the lower production costs realized by the manufacture of continuously deposited, amorphous solar cells, has made photovoltaic systems more realizable. Photovoltaic modules are being commercially used today to power devices such as radios, to trickle charge batteries in parked cars, and in night illumination systems.
Photovoltaic arrays may be used in a wide variety of additional settings. In general, photovoltaics may be used remotely with specific loads (e.g., signal repeating towers) or with unspecific loads (e.g., off-grid residences, off-grid villages), or connected to a power supply grid. Further, a photovoltaics supply may be connected to a power grid either on the customer-side or on the utility-side of an electric meter, depending upon who owns the system. The present invention is directed principally to applications concerning function-unspecific loads, primarily, but not uniquely in the context of grid-connected systems and on either side of the meter.
High efficiency power conditioning units (PCUs) are commercially available today to ensure that photovoltaic arrays operate near their maximum power point. These PCUs maximize energy transfer from sunlight to usable AC electricity. Under actual conditions, a 1 kW rated array is typically capable of producing anywhere between 1300 and 2500 kWh per year in the United States depending on local climate and array geometry. By contrast, an ideal generator working twenty-four hours per day would produce 8760 kWh per year per rated kW. The ratio between the photovoltaic array output and this ideal output is referred to as capacity factor. Hence, for a photovoltaic array, the capacity factor typically ranges from 15% to 28%. Electrical power plants derive value not only from energy production (their capacity factor) but also from their capacity, that is their contribution to a utility""s spinning reserve, hence their ability to deliver power on demand.
Overall, the capacity value of an ideally dispatchable power plant is of the same order as the value of the energy delivered by that plant. Hence, the economics of photovoltaics have traditionally been penalized by the fact that no capacity value is considered for this resource. The present invention is thus directed to capturing additional value for photovoltaics (as well as other non-controllable, renewable resources) by increasing or even maximizing effective capacity of the non-controllable power supply when coupled to a power grid.
Briefly summarized, this invention comprises in one aspect a method for increasing effective capacity of a photovoltaic (PV) power supply coupled to a power grid and a variable load so that the variable load is powered by the power grid and the PV supply. The method includes: determining photovoltaic power supply output and current loading of the variable load; determining an expected peak loading (EPL) of the variable load for a defined time interval and ascertaining therefrom a load adjustment threshold; determining a value for a load adjustment parameter from the photovoltaic power supply output, the current loading, and the load adjustment threshold, the load adjustment parameter being defined as the current loading less the photovoltaic power supply output less the load adjustment threshold; and controlling the variable load employing the value of the load adjustment parameter. This controlling of the variable loading is preferably proportional to the value of the load adjustment parameter when the value is within a predefined range. The load adjustment parameter is employed to control the variable load so as to increase the effective capacity of the photovoltaic power supply without any operational feedback control of the PV supply.
In another aspect, a load controller is provided implementing the above-described method. This load controller controls a variable load coupled to a power grid and to a photovoltaic power supply. The load controller includes means for: (i) determining photovoltaic power supply output and current loading of the variable load; (ii) determining an expected peak loading (EPL) of the variable load for a defined time interval and ascertaining therefrom a peak adjustment threshold; (iii) determining a value for a load adjustment parameter based on the photovoltaic power supply output, the current loading, and the load adjustment threshold, the load adjustment parameter being defined as the current loading less the PV supply output less the load adjustment threshold; and (iv) controlling the variable load employing the value of the load adjustment parameter. This controlling of the variable load is such as to increase the effective capacity of the photovoltaic power supply.
To restate, the load controller/method of the present invention enhances the economic feasibility of photovoltaic power plants by increasing their xe2x80x9ceffective capacityxe2x80x9d through minimal, selective control of the load. Depending upon the geographical location, a load controller in accordance with this invention can add up to $50-$100 per kilowatt per year to the value of a demand-side photovoltaic power plant. The load controller/method presented enhances the grid penetration potential of a photovoltaic plant and enhances the acceptability of photovoltaics to utility companies. In effect, the present invention proposes selectively reshaping utility load patterns to provide a better match with photovoltaic power output, thereby increasing the ultimate share and reliability of photovoltaics in a utility power generation mix. This invention thus enhances deployment opportunities for the more environmentally benign photovoltaic industry. Additionally, the load controller/method presented herein could be applied with other environmentally benign, non-controllable but renewable resources, such as wind power generation.